Compositor, can`t see blur effect from different camera angle
You are using an ID mask to assign the blur on a particular material. But the faces with that material are not visible as they are occluded by the rest of the cube.

If the material is not visible, you won't get the blur. 

It seems to me that if you want to get the blur, you have two choices: 

1- use a separate layer and object masks to create the blur.



2- Use volumetric lights (volume scattering) to have an environment in which suspended particles are lit by the light source that is not visible, as it would happen on a room with dust or or haze.